Recent DIII-D advances in runaway electron measurement and model validation

Abstract

Novel measurements and modeling of runaway electron (RE) dynamics in DIII-D have resolved experimental discrepancies and validated predictions for ITER, improving confidence that RE avoidance and mitigation can be predictably achieved. Considering RE formation, first experimental assessments of the RE seed current demonstrates that present hot-tail theories are not yet accurate and require improved treatment of the pellet dynamics. Novel measurements of kinetic instabilities in the MHz-range have been made in the RE formation phase, with the intensity of these modes correlated with previously unexplained empirical thresholds for RE generation. Controlled RE dissipation experiments in quiescent regimes have validated RE distribution function dependencies on collisional and synchrotron damping, both in terms of distribution function shape and dissipation rates. Measurements of RE bremsstrahlung and synchrotron emission are now used in tandem to resolve energy and pitch-angle effects. A resolution to long-standing dissipation anomalies in the quiescent regime is offered by taking into account kinetic instability effects on RE phase-space dynamics. Kinetic instabilities in the 100-200 MHz range are directly observed, though modeling finds the largest dissipation arises from GHz range instabilities that are beyond the reach of existing diagnostics. Kinetic instabilities are also observed in the mature post-disruption RE plateau phase, so long as the collisional damping rate is reduced with low-Z injection. Experiments with high-Z injection find that the dissipation rate saturates with injection quantity, likely due to neutral diffusion rates being slower than vertical instability rates in DIII-D. Considering the final loss, a 0D model for first-wall Joule heating is found to be in agreement with experiment, and controlled access to RE equilibria with edge safety factor of two identifies novel dynamics brought about by large-scale kink instabilities. These dynamics are typified by fast (tens of microseconds) RE loss rates without RE beam regeneration. The above measurements and comparison with theory represent significant advances in the understanding of RE dynamics and indicate possible new opportunities for RE avoidance or mitigation via kinetic instabilities.

Tomographic reconstruction of the runaway distribution function in TCV using multispectral synchrotron images

“…Strong experimental evidence of such events backed up by kinetic simulations has been reported for DIII-D [26,29]. For the present experiments, this mechanism does not directly explain the increase in population towards the edge of the plasma at high pitch angles and momenta.…”

MeV range particle physics studies in tokamak plasmas using gamma-ray spectroscopy

“…The concern is that a localized loss of the RE beam may damage the tokamak first wall, which is a major risk for plasma operations. Mitigation techniques, predominantly based on massive gas injection (MGI) or shattered pellet injection (SPI) have been developed and are being tested on mid and large-scale tokamaks worldwide [32,33,34] with the aim of preventing the development of a large RE current which would be unbearable for the integrity of the device. In this context, GRS measurements of the HXR bremsstrahlung spectrum born from the REs are key, as they provide access to the RE distribution function and how it responds to the external actuators.…”

Self-consistent simulation of resistive kink instabilities with runaway electrons

“…It can alter the macroscopic magnetohydrodynamics (MHD) stability conditions and thus plays an important role in the disruption process. Several present-day tokamaks, including DIII-D [4], JET [5], ASDEX [6], and J-TEXT [7] have been used to test RE avoidance and mitigation strategies in order to control this issue in ITER and future fusion reactors.…”